Method and apparatus for measuring the earth&#39;s total magnetic field vector



SCHONSTEDT 2,687,507 METHOD AND APPARATUS FOR MEASURING THE EARTHS TOTAL MAGNETIC FIELD VECTOR Filed Sept. 29, 1950 Aug. 24, 1954 7 Sheets-Sheet 1 M Arfrs it 1 PG mm o o 1 mm 2 em a m OO a 0 00 00 W M OA\/ O IIIHIW 1 IHIIHI RN o o 5 T; 5% mm fww mm m@ Nm K OE Aug. 24, 1954 .o. H NSTEDT 2687507 METHOD m5) 15m? FOR MEASURING THE EARTHS TOTAL MAGNETIC FIELD VECTOR Filed Sept. 29. 1950 7 Sheets-Sheet 2 FIG. 3.

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RM ATTYS A g- 24, 1954 a. o. SCHONSTEDT METHOD AND APPARATUS FOR M 2,687,507 EASURING THE EARTHS TOTAL MAGNETIC FIELD VECTOR -7 Sheets-Sheet 6 Filed Sept. 29, 1950 mOmDOm O mKwFuE 0 mohowhuo um Im mmijmze O.

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METHOD AND APPARATUS FOR MEASURING THE EARTHS TOTAL MAGNETIC FIELD VECTOR Filed Sept 29 1950 7 Sheets-Sheet 7 w 535.7 uas n INVENTOR. .E, O. SCHONSTEDT BY Patented Aug. 24, 1954 rs'r LOFF I CE METHOD AND APPARATUS FOR MEASURING THE-.EARTHS TOTAL MAGNETIC FIELD VECTOR Erick o. Schonstedt, Washington, D. o. Application September '29, 1950, Serial N0.:1817,653

' IClaim.

'(Granted-under-S'JiitIe 35, U.=S. Code (1952),

asec. 266) The presentinventionrelatesto a-methoda apparatus for measuring and recordingthe earths total magnetic fieldvector.

Related prior art devices such,"for example, as Patent No. 2,427,014.- issued'to Vv'. J. *-Means-for orienting Device; have been employed formeasuring'and recording the intensity'of the total magnetic field for the purpose Oflccating magnetic bodies iroman aircraft in flight.

and records data. from whicharecomputed-the magnetic heading ofthe aircraft,'horizontal, vertical, the north, and east components 'and the angles of inclination and declination of the earths total magnetic field vector. In other words, continuous recordings are made,"during the flight of an aircraft containingthe apparatus of the present invention, of theintensity of the total magnetic field, the true heading of theaircraft as provided a directional gyro, thexangular displacements about the gimbal axes'of the detector head employed to detectthe magnetic field, variations of attitude of the aircraftand apparatus from true vertical with respect to the earth, and the relative bearing of' the aircraft with respect to the suns azimuth from which the true heading of the aircraft is obtained.

As aforementioned the apparatus of the present invention measures the intensity of the totalmag- .netic field and correlates such information with measurements of the angular position of .theaxis of i the magnetometer detector element with respect to the aircraft in which it is mounted, whereby the total field is definedby components in a rectangular coordinatesystem fixed with respect to the aircraft and including the longitudinal centerline of the aircraft.

It will be understood, of course, that While the longitudinal centerline. of the-aircraft is selected as a referencein the coordinate system, anyother line defined by a particular position of the detector head may be. selected as thereference. .-In

practice, the detector head is adjusted soras. to bring the longitudinal centerline thereofinto parallelism with thecenterline of the aircraft where- 'by the latter may be utilized as a reference in the rectangular coordinate system.

' 'The apparatus of "the present invention also measures the angular position of the aircraft with respect to true vertical "whereby thetotal fieldintensity is defined'by components in a rectangular coordinate system"including true "verti- 2 cal -.and;x.the horizontal component of the. longitudinal centerline.

In order todetermine true heading, whereby the total field intensity is defined by components in a rectangular coordinatersystem including true -north"and-"truevertical, a pendulum stabilized *sunbompass is provided Which: is mounted in theastral domev of the aircraft/The sun'compasshasassociated therewith a camera whereby photographs of the dial readings thereof are made at timed .intervalsythusproviding arecord from which, together with the 1so1ar time sand -graphic position; the; true "headin o f the aircraft-may be computed.

Asv pendulum stability is: affected: during turns, a continuously recording? directional gyroscope is provided for measuring and recording'theheadof *the' aircraft when forces developed durin turns are of- -suficient amplitude to adversely I affect the accuracy of the sun compass.

1 A -recording is madeof the 'variations in the directionof travel-or headingof' the aircraft as evidenced by the directional gyroscope by employing potentiometer havin -its wiper arm attached to the opcrated -shaft ofthe gyroscope. The

potentiometer forms a part ofa bridge circuit,

the unbalance of which is recorded by a recording milliammeter.

=Re2idings of the'angular position of the magnetometer clete'ctor'element are provided by a pair 'ofpotentiometers mounted about the two axes of the gimbal mounting of the detector element to indicatethe angular variations thereof about 'theseaxes; such indicationsbeing recorded on tape by graphic milliammeters individual to and closed circuit'relation with the potentiometers.

The position 'of'the aircraftwithrespect; to vertical is measured by providing anoil-damped gimbal mountedpendulum having a 'pair of potentiometers connectedabout the gimbal axis thereofin such a manner astomeasure'the angular variations'-'of";theaircraft with respect to the pendulum; these potentiometers alsobeing the aircraft 'averages'approximately zero; "During-p'eriods when the aircraft accelerates in pitch and roll directions the deviations can be measured by means of a vertical reference gyroscop or in regions where the earths magnetic field assumes high dip angles, by means of the measurements of the angular motion of the detector coil with respect to the gimbal mounting,

In this manner the angular position of the magnetometer with respect to a true vertical from the earth is made known throughout the entire period of a survey flight, and as herein before stated astral readings are measured and recorded at regular intervals to ascertain the aircraft heading and under some circumstances to fix the position of the magnetometer with respect to latitude and longitude. It will, thus, be apparent that there is provided a system which accuratel measures and records data indicative of the angular position of the magnetometer with respect to the earth at all times during a survey flight, thereby providing data from which are computed accurate values of components of the total field intensity and the direction of the earths magnetic field at all points along the line of flight.

The present invention is of advantage in securing data relating to the properties of the earths magnetic field which are of value in navigation, geophysical prospecting and specialized fields of development and research in terrestrial magnetism. Since there are few regions on the earths surface where the compass needle points true north, it is important in securing accurate measurements for use in navigation, for example, to know the degree of deviation of the reading of the compass needle from tru north in any given region. It is, also, important in the use of magnetic instruments in geophysical surveys where the nominal values of the earths magnetic field are employed to correct the results of such surveys in view of the regional gradient in order to more accurately define the anomalies in the earths crust in the region being surveyed. It is, likewise, of importance in the degaussing of ships and in the design of magnetic mines to have a knowledge of the regional values of the earths magnetic field.

The resulting information secured by the system and apparatus of the present invention may readily be assembled in the form of a chart, such chart being of great bene t in arriving at a correct reading of the earths total magnetic field vector in any of the foregoing problems relating to the earths magnetic field.

An object of the present invention is to provide a new and improved method of determining values relating to the earths total field vector from an aircraft in flight, which values heretofore have been obtained by measurements made on the ground.

Another object is to provide new and improved airborne apparatus for measuring and recording data from which to compute values relating to the earths total field vector.

Still another object of the invention is to provide a method and apparatus for measuring and recording the intensity of the earths total magnetic field and simultaneously therewith for measuring and recording data from which to compute the dip angle and the angle of declination or degree of variation of the magnetic heading with respect to true north.

A further object is to provide a system and apparatus for measuring and recording the intensity of the earths total magnetic field and simultaneously therewith for measuring and recording data from which to compute the vertical and horizontal components of the field.

A still further object is to provide means for recording and coordinating a continuous magnetometer reading of the total magnetic field of the earth from an aircraft in flight with the angular position of the magnetometer in relation to the earth and with the precise heading of the aircraft at all times during such flight.

A still further object is to provide a new and improved system for quickly and accurately measuring and recording data relating to the earths total magnetic field vector over a large area.

Still another object is to provide a system and apparatus for measuring the earths total magnetic field vector by which tape recordings of a plurality of measurements relating thereto are coordinated to produce a chart indicative of such phenomena.

An additional object resides in a new and improved method of converting measurements of the earths total field vector to components in a rectangular coordinat system fixed with respect to the aircraft, converting the aircraft components to components in a rectangular coordinate system taken with respect to the aircraft heading and having a true vertical coordinate and, thereafter converting the last mentioned components to components in a rectangular coordinate system including true vertical and true north.

Still another object is to provide a method of determining true vertical from an aircraft in flight by averaging variations in the aircraft attitude, as measured and recorded by a pendulum,

over spaced intervals of short duration.

Other objects and many of the attendant advantages of this invention will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein:

Fig. 1 illustrates generally and in diagrammatic form the apparatus employed in the present invention and mounted in an aircraft;

Fig. 2 is a top plan of the magnetometer apparatus employed in the present invention;

Fig. 3 is a face view of the control panel;

Fig. 4 is a side elevation of the pendulum apparatus of the present invention and showing the vessel in section;

Fig. 5 is a top plan view of the pendulum apparatus;

Fig. 6 is a schematic vertical gyroscope;

Fig. 7 is a schematic perspective view of the directional gyroscope;

Fig. 8 is a schematic diagram of the electrical circuits of the present invention;

Fig. 9 is a schematic perspective of the magnetometer assembly;

Fig. 10 is a diagram of the pendulum mechanism and showing the orientation thereof with respect to the magnetometer assembly;

Figs. 11, 12, 13 are diagrams showing the projection of various components of the earths total field vector with respect to the three coordinate systems employed in the present invention;

Fig. 14 is a diagram illustrating in greater detail one of the bridge circuits for use in the recording system for indicating angular deviations of the detector coil;

Fig. 15 is a schematic diagram of the photographically recording sun compass employed with the present invention and showing the means for perspective view of the operating the recording mechanism therefor; and

Fig. 16 is a diagram illustrating the relative bearing of the aircraft with respect to. the sun and the aircraft heading with respect to true north.

Referring more particularly to the drawings wherein like numerals indicate like parts throughout the several views, an aircraft capable of long periods of sustained; smooth flight is indicated generally at I in which are mounted a magnetometer assembly II, a control panel I2, a plurality of recording oscillographs indicated generally at I3, a pendulum assembly I4, a sun compass I50, and other associated apparatus which will be more thoroughly described hereinafter.

The magnetometer assembly II generally follows the construction of that disclosed in the aforementioned Patent No. 2,427,014 issued to W. J. Means for OrientingDevice, and is provided with a rectangular nonmagnetic frame I5. Frame It is supported by and secured to a base member I which is, in turn, supported by shock absorbing mounts I I in a recess I8 formed in the lower portion of the fuselage of aircraft I0. The position of recess I8' within the aircraft I0 is such that a compromise between closeness thereof to the center of motion of the aircraft and the. greatest possible distance thereof from the sources of magnetic disturbanc such, for example as the engines, is effected. It is also desirable to locate all apparatus in the aircraft which produces a strong magnetic field in such a manner as to have th least possible eifect on the magnetometer II.

As indicated in Fig. 1, a position slightly aft of amidship has been found most desirable in the present installation. It is, of course, understood that the most favorable position for the magnetometer will vary in different types of aircraft.

A housing I9 is provided for three mutually perpendic ular magnetometer elements 2 I, 22, and 23. Element 2I is an axial magnetometer; 22 is a transverse magnetometer, and 23 is the detector magnetometer. Thedetector magnetometer 23 constitutes the physical element to be oriented in the magnetic field to be measured. The magnetometer elements 2 I, 22 and 23 are each excited by voltages of fundamental frequency from an A. C. source, axial magnetometer element 2| and transverse magnetometer element 22, which form a part of the orienting system for element 23, being arranged in such manner that when they form an angle other than 90 with respect to the direction of the magnetic field to be measured. there is induced therein a second harmonic voltage of magnitude and phase depending upon the magnitude and direction-of angulardisplacement.

The detector element 23 as well as elements 2I and 22 of the magnetometer are saturable inductors, each comprising a coil of several thousand turns of fine wire and a core constructed of a thin strip of highly permeable metallic alloy rolled in scroll form. Each coil is excited by the application of a 1000 cycle A. C. current of sufiicient amplitude to drive the core thereof into saturation on the positive and negative peaks of each cycle. The rise and fall of the flux in the core produced by the excitation current induces a voltage in the coil which is proportional to the rate of change of the flux in the core. When no external magnetic field is applied along the axis of the core other than the field produced by the excitation current, the wave form of the induced voltage will contain only odd order harmonics, i. e., 1000, 3000,5000 cycles, etc. When the core is magnetized by a weak field such, for example, as a component of the earths magnetic field, this bias will shift the operating point of the 1000 cycle current and cause it tosaturate the core for a greater length of time on one'peak of its cycle than on the other. Asa result, the wave form of the induced voltage produced by the rise and fall of the fiux in the core will become asymmetrical. This asymmetrical wave form,- therefore, will contain even order harmonics, i. e., 2000, 4000, 6000 cycles, etc. which over a sulficiently useable range'of magnetic field'values will be proportional-tothe magnetic field along the axis of the core. The 2000 cycle harmonic'which is isolated by means of an accurate 2 kc. band pass filter is employed for measurement purposes.

Since the earths magnetic field ranges from about 25,000 gammasto 70,000 gammas (one gamma equals 4 oersted) small variations of the earths magnetic field cannot be measured simply by means of an electrical meter. There-- fore, a very accuratelymeasured direct current is passed through the detector coil for nulling out the greater portion of theearths magnetic field therein. This current is regulated by means of control box I00 with which the nulling current may be varied by means of switches 88, 89, 9| in increments corresponding to 5000, 500 and 50 gammas change of magnetic field respectively. Since the earths magnetic fieldintensity does not go below about 25,000 gammas, the control box provides for changing the nulling field from a base value of 22,000 gammas to 72,000 gamma-s. Variations in total field intensity about a given control box setting are recorded on meters 8i and 04. The total magnetic field intensity measurement then consists'of the base value, the

set on 7 to null 35,000 gammas, dial 89 on 4 to null 2000 gammas, dial III on 6 to null 300 gammas, and gammas read on meter 8| or 84, the control box base value being 22,000 gammas, the total intensity would be or a total of 59,385 gammas.

Variable resistor I38 connected in series with meters 8I and 84 is adjustable in steps which provide for full scale readings of the meters of 50, 100, 200, 500, 1000 and 5000 gammas.

The housing I9 is formed of a solid piece of suitable plastic material and is provided at the ends thereof with pivot bearings for receiving mounted in inner gimbal ring 25, which is supported by pivot bearings for receiving pivots 20 mounted in outer gimbal ring 21 and arranged at from pivots 24. The outer gimbal ring 27 is fixedly secured to frame I 5. It is understood that in order to provide for electrical connections between the magnetometer elements 2I, 22and 23 and frame I5, slip ring assemblies 28, 29 and (H are employed, slip ring assembly 23, comprising four rings, being arranged for electrical conduction between housing It and inner gimbal ring 25, while slip ring assemblies 20 and 3I, comprising two rings each, are arranged for electrical conductionbetween inner gimbal ring 25and outer gimbal ring 21 A pulley 32 is formed on-housing I 9 intermedi ate the ends thereof and at right angles to the axis through the pivots 24. An endless cord 33 has a portion thereof looped about pulley 32 and passes over suitable pulleys 31 to align it with the axis of pivots 26 each of which have bores formed therethrough, the cord passing through the bores to the exterior of the outer gimbal ring 21.

Mounted adjacent the end of frame opposite to that occupied by the magnetometer housing I9, is a pair of two-phase electric motors 34, each motor having a reduction gearing 35 associated therewith, each of the driven shafts thereof having a double drive pulley 36 thereon. The cord or belt 33 passes over suitable pulleys 31 mount-ed on frame 15 to direct it toward the drive pulley 36 of a first motor 34.

A pulley 38 is fastened to inner gimbal ring 25. Endless cord 39 has a portion thereof looped about pulley 38 and passes over suitable pulley 4| to the drive pulley 36 of the second motor 34. Each of the drive pulleys 36 has associated therewith a yieldable belt tightening apparatus 42 to ensure adequate tension on cords 33 and 39 for an accurate slip-proof drive.

Axial and transverse spirit levels 43 and 44, respectively, are mounted on the outer or fixed gimbal ring 21 to establish true level of the frame with respect to the aircraft.

The oil-damped pendulum indicated generally at I4 may be mounted on the floor of aircraft I!) or other convenient location. The pendulum assembly comprises a base 46, adjustably and vibration absorbingly mounted in aircraft ID as at 4! and having mounted thereon a pair of spirit levels 46 and 49, the level 48 being axially mounted with respect to theaircraft and level 46 being mounted transversely thereto. A pair of spaced vertical frame members 5| are mounted on base 46 and extend vertically therefrom. A cross member 52 is mounted on the upper extremities of members 5| and forms a support for the gimbals or universal joint assembly generally indicated at 53. Depending from the joint 53 by means of rod 54 is an inertia member 55. A vessel in the form of a cylinder 56 is mounted on base 46 between frame members 5|. The inertia member 55 depends into cylinder 56 to a point adjacent the bottom thereof. Silicone oil 51 of suitable viscosity is placed in cylinder 56 to damp the movement of inertia member 55. Potentiometers 58 and 59 are mounted on gimbals assembly 53 in such a manner that movement is produced between the resistor element and the contact arms of the potentiometers as movement of the inertia member 55 occurs in a direction longitudinally of the aircraft to indicate pitch in the case of potentiometer 58 and in a transverse direction to indicate roll in the case of potentiometer 59.

A reading of the variations of current passing through potentiometers 58 and 59 as the inertia member moves in response to tilting of the aircraft is presented in tape form by a pair of recording oscillographs, as will be hereinafter more fully described.

The spirit levels 48 and 49 are employed as a reference in conjunction with levels 43 and 44 in order to bring the base 46 into perfect parallelism with the frame I5 and the magnetometer assembly H by adjustment of the mounting arrangement 41.

In the use of the present invention, the mounting for the magnetometer H and the pendulum l4 being parallel to the longitudinal axis of aircraft l6, angular motion of the airplane in two mutually perpendicular directions is recorded on the recording oscillographs or milliammeters 6| and 62 actuated by variations of current in the control circuits therefor caused by changes in resistance of the potentiometers 58 and 59 as they are moved by action of the pendulum member 55. It will, thus, be apparent that the angular positions of the aircraft and the apparatus secured to and contained therein may be recorded throughout the flight of the airplane.

It is also to be noted that the positions assumed by the detector coil assembly of the magnetometer during the flight will be recorded on recording oscillographs or milliammeters 63 and 64, which are actuated by variations of current in the control circuits individual thereto.

The control circuit for recording milliammeter 63 and milliammeter 85, which records the axial inclinations of the detector coil 23, comprises a bridge circuit 13 having the resistor of potentiometer 65 across an A. C. source. There is also provided a resistor 14 across the A. C. source, a variable contact arm 66 being associated with the resistor of potentiometer 65 and manual control arm 15 being associated with resistor 14 on control panel l2. Leads from arms 66 and 15 are connected to the input of a phase detector and amplifier 16 while the meters 63 and are connected to the output thereof.

While the resistors 14 and 18 have been shown in Fig. 8 as being single resistors because of space limitations, these resistors are shown in greater detail in Fig. 14. It is apparent from Fig. 14 that the potentiometer 65 which is similar to 61 comprises a continuous resistor having a spread of 45 plus and minus of 0. Potentiometer l4 which is similar to 18 comprises a plurality of resistors in increments of 5 and has a 90 spread, 45 plus and minus of 0.

The circuit for recording milliammeter 64 and milliammeter 86 is similar to that employed for meters 63 and 65, there being provided a bridge circuit 11 having the resistor of potentiometer 67, across an A. C. source. Potentiometer 67 is provided with contact arm 68. Resistor 18 is, also, across the A. C. source and is provided with a variable contact arm 19. Leads from contact arm 68 and arm 19 enter the input of phase detector and amplifier 80, meters 64 and 86 being connected in the output circuit of 80.

It is, of course, understood that potentiometer 65 is mounted on outer gimbal ring 21 and contact arm 66 is mounted on the inner gimbal ring 25 whereby axial movement of the detector coil 23 will cause relative motion therebetween. The potentiometer 6'! is mounted on the inner gimbal ring 25 and its contact arm 68 is mounted on housing l9 whereby transverse angular motion of the detector coil 23 causes relative motion therebetween.

In order to compensate for the magnetic field of the aircraft, and to prevent disturbance of the magnetic detector circuit thereby, three mutually perpendicular compensating coils 69, II and 12, Fig. 2, are provided, the coil 69 being aligned along the axis of housing 19, the coil H being aligned transversely thereof, and coil 12 being wound around the outer gimbal ring 21. When suitable voltage is applied to the aforementioned coils it is clear that the effects of the permanent type magnetic field of the aircraft and magnetic equipment contained therein may be effectively nullified.

Milliammeters 84, 85, and 86 associated respectively with recording milliammeters ill, 63, and 64, are mounted on the control panel l2 in order to be in view of an operator. Step resistors 88, 39, and d of the nulling control box I00 for the detector are also mounted on control panel I2 and are arranged in the circuit of detector coil 23. Variable resistors 88, 09, and 9! are provided with a series of contacts for changing the nulling field in the detector in increments of 5000, 500, and 50 gammas respectively. Each of the resistors has a pair of banks of step resistors, one bank of resistors being connected in series through a selector arm 90 with battery 92 and one end thereof being connected to detector coil 23, while the other bank of resistors is connected to ground at one end and to battery 92 through the selector arm 90. As more resistance is applied to the detector coil circuit by adjusting resistors 88, 09, 9!, less resistance is connected to ground or vice versa. In this manner drain on battery 92 is substantially the same regardless of the amount of resistance connected to the detector coil in the field biasing circuit therefor. The purpose of applying this nulling current to the detector coil, as aforementioned, is to substantially nullify the effects of the normal earths magnetic field on the circuit thus thetotal field strength is read above a known base value by combining the readings of resistors 80, 89, and 9| and those of meter 8! or 8 This field biasing circuit arrangement is generally similar to that disclosed in the copending application of E. P. Felch, Jr. et al. for Detection System, Serial No. 618,550, filed September 25, 1945.

A milliammeter 81 and variable resistor 90 are arranged in the aforementioned circuit in order to provide for the accurate adjustment of the output of battery 92. In the diagram of Fig. 8 the foregoing apparatus is enclosed in the box I00. The foregoing apparatus I00 is connected in parallel with the detector coil 23 and a one kilocycle A. C. oscillator I42 having a one kilocycle filter I43 associated therewith. As aforesaid, the coil 23 is excited by the 1000 cycle A. C. current, supplied by the oscillator I02, to drive the core of the coil into saturation at the positive and negative peaks of each cycle which induces a second harmonic signal voltage in the coil. This signal voltage passes through a 2 kilocycle filter I44, A. C. amplifier and rectifier I45, D. C. amplifier I46, to meters 84 and 8! thus producing a reading on the meters as has heretofore been described. The schematic diagram of an electrical circuit for the recording milliammeters 6 I and 62 associated with pendulum I4 for indicating pitch and roll, respectively, of the aircraft is, also, illustrated in Fig. 8. Mounted in series with recording milliammeter 6I is a milliammeter 93, while milliammeter 96 is mounted in series with recording milliammeter 52. A bridge circuit 90 is arranged in the circuit of recording meter 6!, while bridge circuit 91 is arranged in the circuit of meter 02. Potentiometer 58, which is mounted on the pendulum gimbal 53 to indicate pitch, comprises two arms of the bridge 90, the other arms comprising resistors 98 and 99. When the aircraft is horizontal with respect to the earth, rod 54 is vertical and contact arm I0! of potentiometer 50, which is connected in series with meters 6| and 93, is in the mid position of the resistance element thereof, the ends of the resistor being respectively connected to a D. C. source. There is provided a fixed connection I02 to meters 5! and 93 between resistors 98 and 99, the outer ends of the resistors also being connected to the D. C. source. In this position of contact arm IOI, the circuit for meters 6! and 03 is balanced and the meters read zero. As the aircraft changes angle from the horizontal along the azis thereof the contact arm I0! moves from the mid position thus causing an unbalance in the bridge which, in turn, causesthe meters to move from their zero positions a distance proportional to the movement of contact arm I0! and in a direction depending upon the polarity of the unbalance, the bridge, as aforementioned, being connected to a source of direct current.

The bridge circuit 91 is similarto bridge circuit 90, comprising the potentiometer 59, which is mounted on gimbal 53 to indicate roll, forming two arms of bridge 91, the other arms comprising resistors I03 and I0 3. Potentiometer 59 is provided with a movable contact arm I05 which contacts the resistor of the potentiometer at the mid point thereof when the rod 54 is vertical and the aircraft is horizontal. There is, also, a fixed connection I06 between the resistors I03 and 900. The ends of resistor of potentiometer 59 and the outer ends of resistors I03 and W4 are connected to a source of direct current, while the arm I05 and the fixed connection I00 are respectively connected to the two sides of the circuit of meters 62 and 06. It is apparent that the operation is similar to that of the circuit controlling meters 6! and 03 as heretofore described, the unbalance caused by movement of arm I05, in turn, causing the meters to indicate the degree of roll of the aircraft. The pendulum 45 has a limited movement of 5 in any direction from the zero position therefore it is not necessary to provide variable resistors as in the foregoing bridge circuits 13 and 11.

An aircraft in fiight has a tendency to develop oscillations which vary from true horizontal flight there being short alternate periods of climbing and descending along the path of travel of the aircraft, such oscillations being called phugoid oscillations. In employing the pendulum ef the present invention the effects of such oscillations are substantially reduced by averaging two cycles of the oscillations. There are, also, present during flight of the aircraft transverse oscillations, each of much shorter duration than the foregoing oscillations of the longitudinal axis, these shorter oscillations are, also, averaged for a period of time equal to the foregoing two cycles, it being understood that due to the shorter period of each oscillation there are many more cycles present and the magnitude of error in reading is very small.

The action of the pendulum oscillations is as follows:

As the aircraft climbs, there is a corresponding loss of forward speed which causes the pendulum through inertial force to move forward thereby tending to counteract the effect of change of angle of the aircraft on the pendulum. As the aircraft descends, there is a corresponding increase of forward speed which causes the pendulum through inertial force to move to the rear thus tending to counteract the effect of this form of change of angle of the aircraft on the pendulum. It will, thus, be seen that during such oscillations of the aircraft the reading of meters 0!, 62, 93 and 05 will be somewhat inaccurate, such in accuracy being substantially nullified by the aforementioned averaging of two cycles of such oscillations.

During acceleration or deceleration of the air craft, the readings produced by the pendulum It will also not be entirely accurate, therefore a vertical gyroscope 401, Fig. 6, is provided having a measurable tilt of 5 in any direction from vertical during the phugoid 11 and mounted in a gimbal I08 which is provided with potentiometers I89 and H8 which will indidate pitch and roll, respectively, of the aircraft. This gyro determines the period of the phugoid oscillation for obtaining accurately the periods over which the data are averaged.

As illustrated in Fig. 8, the circuits provided for recording milliammeters III and H2 and milliammeters H3 and H6 associated with gyroscope I01 are arranged in the same manner as the circuits for meters GI, 62, 93, and 96. A bridge circuit H5 is provided for meters III and H3, while bridge circuit H6 is provided for meters H2 and H4. Potentiometer I09 of bridge H5 is connected at its ends, as are the outer ends of resistors H1, H8, to a source of direct current, while a variable contact arm H9 associated with the winding of potentiometer I09 is connected to the gimbal mounting I08 in such a manner as to indicate pitch of the aircraft by causing an unbalance in the bridge circuit I I5 when the longitudinal axis of the aircraft moves angularly with respect to the horizontal, the arm I I9 being connected to one side of the meter circuit, while the inner ends of resistors H1, H8 are connected to the other side thereof.

The potentiometer H is connected in bridge circuit H6 of meters H2 and H4 and has a variable contact arm I2I connected to one side of the meter circuit, while the outer ends of the resistor of potentiometer H8 are connected to a source of direct current. Resistors I22, I23 have their outer ends connected to the source of direct current, while the inner ends thereof are connected to the other side of the meter circuit. The

arm I2I is connected to the gimbal mechanism I08 in such a manner as to cause an unbalance in the bridge circuit H6 to indicate roll when the aircraft varies angularly from the horizontal in a direction at right angles from the longitudinal axis of the aircraft. The vertical gyroscope I31 is mounted with the outer gimbal axis thereof in parallelism with the longitudinal axis of the aircraft.

In order to make astral readings indicative of true heading and geographic location in connection with the present invention, there is provided a sun compass or relative bearing indicator dis closed in my copending application for Sun Compass, Serial No. 214,619, filed March 8, 1951, now

Patent No. 2,612,693, and indicated generally at I50. As indicated in Fig. 1, the sun compass I50 is mounted in the astral dome II of aircraft I8.

As illustrated more clearly in Fig. 15, the compass I comprises an adjustable structure I52 for rotatably supporting a vessel I53 containing 5 oil I54, a gimbal I55 mounted on cover I56 of vessel I53, a shaft or rod I51 extending into vessel I53 and supported by the inner ring of gimbal I55. The rod I51 has mounted on the lower end thereof a mass I58 which is normally immersed in oil I55 and motion of the mass is damped thereby. Rod I51 has a vertical extension projecting a distance beyond gimbal I55.

That portion of gimbal I55 which supports rod I51 also supports a horizontal disk I59 formed of a transparent material and having a semi-trans parent covering and a scale I6I at the periphery thereof, the scale being marked in degrees of azimuth. In operation, the shadow of the projecting extension of rod I51 falls across scale ISI, thus providing a reading of variations of heading and geographic position of the aircraft with respect to the sun.

A record of the aforementioned variations in heading is made by photographing at regular intervals the shadow of the rod I51 as it appears on scale I 6I of disk I59. A camera I62 mounted for rotary adjustment about the vessel I53 is provided for producing such photographic record. The camera I62 may be of a well-known variety suitable for the purpose and is provided with a means for timing the intervals between exposures such, for example, as the cam and switch arrangement indicated generally at I63.

The cam I64 of this timing means makes one revolution in five minutes and has a plurality of raised cam surfaces I65 to close switch I66 whereby operating mechanism I6! is energized to expose the film contained in the camera and move it to the succeeding position every 6 seconds for a period of approximately two minutes of each five minutes of operation which provides an inactive period of approximately three minutes between each two minute period. Preferably the cam I64 is provided with two or more additional c m surfaces I65 at each end of the two minute period to permit adjustment of the two minute time period where desired.

The cam I64 is driven by a synchronous motor in the power circuit for driving the recorders I3 and having reduction gearing to reduce the speed sufficiently to produce the aforementioned result.

Pendulum stability is affected during turns of the aircraft, and for this reason a directional gyroscope I24, Fig. 7, is provided to determine when such turns occur which will adversely affect the accuracy of the angle measurements of the pendulum and sun compass. Gyroscope I24 has associated therewith recording milliammeter I25 and milliammeter I26, Fig. 8. A potentiometer I21 is provided with a resistor element I28 which has a 200 degree spread and a pair of separate contact arms I29, I3I spaced degrees apart. The ends of resistor element I28 are connected to a source of alternating current. A circular array of contacts I32 are arranged at 10 intervals from 0 to 350, the contacts from 10 to 180 being interconnected and connected to arm I29 of potentiometer I21. The contacts from to 0 are interconnected and contact arm I3I of potentiometer I21 is connected thereto. A contact arm I33 engages individually the contact I32 as it is rotated and is connected to one side of the phase detector and amplifier I30.

A plurality of step resistors I36 are arranged between contacts I35 of variable resistor I36. Contacts I35 are spaced 10 degrees apart from 10 through 180. A pair of resistors I31 are connected between the 10 contact and one side of a source of alternating current. A single resistor I38 is connected between the contact at 180 and the other side of the A. C. source.

The resistors I34 are arranged in approximately half of a complete circle and a pair of contact arms I39 spaced 180 apart are provided in order to effect the functional equivalent of a complete ring of resistors and thus avoid placing an excessive amount of resistance in the circuit.

The resistors I28 and I34 comprise arms of a bridge circuit similar to the foregoing bridge circuits 13 and 11. As will be apparent, if 0 is the true heading and the aircraft is 3 to the left of the true heading, it being assumed that arms I33 and I33 are at the 0 setting, the resistor I38 is in one side of the circuit, while the portion of resistor I28 between 177 and 190 is effective to cause an overbalance of resistance the equivalent of 3, thus there will be a deflecaesa'soas tioniof the. needles "of meters I2IandI26 the1v needles thereof within this 10 limit the resistors I 34 are provided. its the contact arms I29, I3I move beyond the first 5 (plus or minus) from the 0 position of resistor I23, the interconnected contact arms I 33, I39 are moved by a single control knob I49. associated with dial I litothe 10- setting, in case of a plus 5 reading of the meters, or to the 350 setting, in case-of-a minus 5 reading. It will thus be apparent that the bridge circuit is balanced within 10 bymov ing the contact arms I39, I33 to the nearest 10, the difference being readable on the meters I26. Asthe directional gyroscope has a full 360". turn, the arms I 29 and HI being moved in unisontherewith and as resistance I23 is only. provided between350 through 0 to 190 reading in a clockwise directiongthe. bank of contacts I 32 with contact arm I33 :are provided to-shift the circuitfrom arm I29 to arm I3! or vice versa' as one or the other arm moves off the resistor I 23.

The direct current meters I25, I26 are actuated by passing the output current of the bridge cir cuit comprising resistors I27 and I St through the phase detector and amplifier I30.

The recording tape of meter I 25, as are .the tapeszof all of therecording milliammeters I 3 and the camera for sun compass I58 are driven by synchronous motors connected to a common source of accurately 1 cycled alternating current; In order to synchronously mark the edges of the tapes of recording milliammeters I3 with:

indications of time or other indicia, each of the recorders is provided with-right and left edge markers I II and H2, respectively. These edge markers are operated electrically by suitable right and left circuits (not shown), the circuits being closed by pushbutton switches I13 and I'M, respectively, mounted on control panel I 2.

The process by which the direction of the total magnetic field Vector is determined with respect to geographic coordinates-is carried out in'the following three steps:

(a) The direction of the earths total magnetic field vector is determined with respect to a rectangular coordinate system which is fixed with respect to the aircraft: andincluding. the longi-- tudinal axis or centerline thereof as on'e'of "the coordinates.

(b) The attitude :of .the aircraft is determined with respect to a second-rectangular coordinate system including the horizontal componentof the aircraft centerline and true vertical.

(c) The attitude of the aircraft is determined. with respect to a third rectangular coordinate system including true north.

The result under (a) as aforementioned is accomplished by measuring the angular rotation of the detector coil about the two axes of the gimbal in which it is mounted. These angles are measured by means of bridge circuits 'ifl'and "I? which include the potentiometer-s 65 and: I5]; respectively, mounted on the gimbal axes'as shown in Figs. 2 and 9. As shown in Fig. 9,v the gimbal ring 25 rotates on axis A and the angle of rotation about this axis is designated a. Housing I3 rotates on'axis B and" the .angle' of rotation about this axis-is designated-6. Angles a and 3 are considered to be at zero when axes-A and B lie in a horizontal plane established bylevels '43 I and 44 mounted on ring 21 which is fixed to frame. I5 and when the. axistC of the detector coincideswiththe:axes A, B and C when and'p equal-zero Inmounting-'mecham'sm II in aircraft Mfr the axis A is arranged horizontal and 'perpendicular to the centerlineofthe'aircraft; Under such condition thecomponents'of the earths magne-tic -field'with respect to the aforementioned" aircraft coordinate system --will be as indicatedih ZA, XA' andYA are,- in this position, the

Fig 11. vertical'and horizontal coordinates "taken with respect to'the aircraft; The verticalcomponent' Fzii is taken positivedownward to comply with the convention in which'the vertical component in the northern hemisphere, whichis' directed "downward; is considered positive.

It should beobserved that the gimbal'mount' ing of the housing I9 which contains the detector coil is such that the axis 13 pivots about axisA, the latter beingfixed with respectto'the' aircraft. The angle or is the angle between XAand axis'B, as shown in Fig. 11. The plane in'whi'ch.

8 is measured is alwaysfperpendicularilto axis 13.. The component FXA in the direction of flight. wi1l.be1

FxA=F cos 9 sin .a (l) where F is the'total magneticfieldiintensity and is parallel. zwithtathe .detector' axis; i The'i'hori-i zontal component:FyA, perpendicular "to the di-i' rection of night is Fm: -F-'sin p (2) The vertical component Fz'ii" is FZA=F cos ,8 cos a (3) The present system is arranged to give the conventional positive polarity to the downwardly directed verticalc'omponentof the earths magnetic demand to employ a conventional right hand coordinate system; It will be note'd in Fig.

11 that the angle-e is developed clockwise about- 1 axis xt and zi'stherefore considered negative-byconvention wherefore the component FYA is positive notwithstanding the negative sine in the equation therefor, the sine of 3 being negative and F being positive.

The pendulum assembly I I, Figs. 4 and 10, is employed as aforesaid for determining true vertical, the potentiometers 58 and 59 reading zero when the two gimbal axes I and 2 are horizontal, the rod 54, axis 3, being vertical. The two levels is and 19 indicate when thependulumis leveled withthese axes horizontal. Although, the pendulum may be mounted in several positions, for this analysis it is assumedthat the'pendulum is carried in the aircraft withthe outer axis I of the imbal 53'para1lel withcoordinate XA'. Axes I and XAwill be substantially parallel with the center lineof the aircraft. This axisis rigidly secured withrespect to the outer frame 'of the pendulum and therefore pitches with 'the' air.- craft. The inner axis 2 .of the'gimbal is-fixed with respect-to the pendulum arm'an'd thus is always horizontal assuming that the aircraft is under a condition of nonaccelerated.flight..l Whileiit is,..of course, perpendicular to the centerline,.it is parallel to .Yxionly. whenrthe latter is truly .-hori-;.

zontal. The. orientation .of ,the detector-and.

pendulum gimbalsris showninFig. 10,

of its coordinates and the projection of the X. coordinate on a horizontal plane as another. The coordinate axes are labeled XE, YE, and ZE, Fig. 12. XE is the horizontal coordinate parallel to the aircraft heading, YE is truly horizontal and perpendicular to the aircraft heading, and Zs is truly vertical. Axes l, 2, and 3 will be parallel to XE, YE, and Zn, respectively when angles p and 'r, which respectively designate the pitch and roll angle of the aircraft, are zero.

Let it be assumed that the aircraft has pitched through angle 10 as shown in Fig. 12. In a pitch motion the outer axis of the gimbal pitches with the aircraft, the gimbal being fixed thereto. Since the coordinate X. is also fixed with respect to the aircraft it is coincident with axis I and moves with it. The amount of pitch motion is registered by reason of the rotation of rod 54 about the inner gimbal axis 2. Since the inertia member remains in a stationary attitude with respect to vertical, it may be stated that with respect to the earth the coordinates XA and ZA pivot about axis 2.

In Fig. 12 it is also assumed that the aircraft has undergone a roll motion through the angle 1. This roll motion is registered by reason of the pivoting of the pendulum inertia member about axis I. Since the inertia member remains in a stationary attitude with respect to vertical, it may be stated that with respect to the earth the coordinates YA and ZA pivot about axis l.

The components measured with respect to the pendulum vertical reference coordinate system are expressed mathematically as follows:

zontal, the rod I51 being vertical. The scale disk I59 is also adjusted so that a line through 0 and 180 thereon is parallel with the centerline of the aircraft, the 0 being forward and the 180 being aft.

By reason of this arrangement, the line I10, which in the diagram of Fig. 16 represents the shadow of rod l5! of the sun compass, makes an angle 0 with respect to the heading of the aircraft. If the heading of the aircraft were true north, line H0 would make an angle with respect to the aircraft heading. The true heading (T. H.) of the aircraft, therefore, is given by the angle 6 where Angle may be determined by means of a computer such, for example, as the azimuth computer disclosed in Patent No. 2,408,776 issued to Joseph Hilsenrath and Samuel Globe, the sun time and latitude being known. Angle 0, of course, is the relative bearing of the aircraft with respect to the sun and is obtained from average readings along time spaced intervals of the photographic record, as aforedescribed.

Angle e is utilized in setting up the third rectangular coordinate system of the present invention as disclosed in Fig. 13 wherein the coordinates comprise north, east and true vertical or nadir.

In the coordinate system of Fig. 13, components FXE and FYE are displaced from the north and east coordinates by the angle 6. Under these conditions the field components X and Y along the north and. east coordinates are expressed FXE=FXA cos p+FyA sin r sin p-l-Fza cos 1" sin 11 5 by the equations:

:F (sin a cost? cos p-s1n 5 S111 r XZFXE cos IE FYE sin 6 (8) Sm p+cos cos cos T Sm (4) :F (sin or cos [3 cos p cos esin 3 sin r FYE=FYA cos T--FZA sin 1' sin 13 cos e+ cos a cos B cos 1 sin 13 cos :F (sin 5 cos r-cos a cos f3 sin 7) (5) e+SiI1 8 cos r sin e+COS 0: cos ,3 sin 1' sin e) FZE=FZA cos 1" cos PFXA sin p+FYA sin 1' cos p Y=Fys cos +FXE sin e (9) =F (cos 0: cos ,8 cos 1 cos psin 0c cos :F (sin 5 cos 1 cos ecos 0: cos ,6 sin 1' [3 sin p-sin s sin r cos p) (6) cos e+sin u cos ,8 cos p sin c-SiIl 8 sin T For very precise results, angle r is corrected Smp E+cs cos 5 cos T Sm Sm for a deflection of the pendulum due to coriolis 5 The declination angle D is expressed by the acceleration. The magnitude of this correction equation:

Y D=tan- Y l0) (-sin 5 cos r cos 6-005 0: cos B sin r cos 6+ sin 0: cos 6 cos '1) sin esin B sin rsin p sin 6+) =tan 1 cos 0: cos 6 cos r S111 7) sin 6 sin 0: cos B cos p cos e-sin 6 sin r sin p cos 6+ cos a cos 8 cos r sin 11 cos e+sin B cos r sin 6+) cos 0: cos 13 cos 1' sin p sin 6 may be obtained from the tables of the Air Declination is defined as the angle between Almanac. The corrected value of angle 7' is true north and the direction of the horizontal employed in computing Fxr, Fys and Fzn. component of the earths magnetic field. There- T e fi al Steps n establishing the direction fore, if the direction of the horizontal component of the magnetic field Vector w t esp ct to W lies in a clockwise direction from the true north g ap c coordinates including e l when viewed from above, the declination is west, prises determmatlons of true headmg, and and if in an anti-clockwise direction, the declinaground position. True heading is obtained by tion is east astral observations using the sun or a star, preferably the sun, as the reference. Ground posiggggfi? dlp ormchnatlo-nhs exmessed by tion is obtained by a variety of means, depending on flight conditions, comprising contact F flying, astral observations, loran fixes and other means suitable for the purpose. +1

As may best be seen in Fig. 10, the sun compass (cos 5 T 005 1" I50 is initially adjusted in a manner similar to Sm cos 6 sm p sm Sm T cos p) the adjustment of the vertical pendulum assembly 14 so that the outer gimbal axis thereof is g z gifgs component H 18 expressed by initially horizontal and parallel to the centerline of the aircraft and the inner gimbal axis is hori- H =F cos I (12) I 7 The vertical component, Z is Z=F sin I=FzE (13) The magnetic heading, M. H. is

F M. H.=

tan FXE =tan 1 +sin 6 cos r-i-cos n: cos 3 sin 1' sin 1; cos 5 cos p-sin 8 sin r sin 12-}- cos cos cos 1* sin p The foregoing equations appear to be somewhat involved, although the functions consist of only simple trigonometric relations. The computations do not become difiicult if the values of the various functions are set up in tabular form and a calculator is employed for securing the answers. If the detector head and pendulum are properly leveled initially, the pendulum angles will be small and cos r and cos p can be taken as unity. Also, the term involving (sin r) (sin 10) becomes negligible.

All of the magnetometer data are continuously recorded except for control box dial settings which are noted on a separate data sheet. In order to obtain a definition on the record chart of less than 0.1 degree in recording the angular data, the aforedescribed bridge circuits are employed.

Alternating current sources are employed for energizing the detector bridge circuits l3 and 11 in view of the close proximity of these circuits to the detector coil, thereby to avoid setting up direct current fields which would interfere with the field to be measured by the detector.

It is apparent from the foregoing description that in employing the method and apparatus of the present invention recordings of the intensity of the earths magnetic field is correlated with the angle of the earths magnetic field with respect to the aircraft containing the apparatus, 1. e., the a and B angles which the detector coil makes with respect to the aircraft, the angle of the aircraft with respect to true vertical, 1. e., the aircraft pitch and roll angles p and r, and the relative bearing angle 6 of the aircraft with respect to the sun. It is of course, understood that the tapes employed in all of the recording milliammeters are driven by synchronous motors at a constant speed, thus all of the various data recorded at a certain time on all of the tapes and on the photographic record of the sun compass is correlated with respect to time. It is also apparent that by means of a series of surveys employing the apparatus of the present invention the information relating to the earths total magnetic field vector may be incorporated in a chart covering a large area of the earths surface.

Charts of this type for world wide coverage are prepared and/or issued by the U. S. Coast and Geodetic Survey and Navy Hydrographic Oflice.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claim the invention may be practiced otherwise than as specifically described.

The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

What is desired to be secured by Letters Patent of the United States is:

Apparatus for measuring and recording the attitude of an aircraft in flight with respect to the direction of the earths total magnetic field comprising, in combination, magnetic detector means for measuring the total intensity of said field including a gimbal mechanism and a field detector element supported by the gimbal mechanism orienting means for maintaining said field detector element continually aligned with the direction of the field by pivotal movements about the gimbal axes, said gimbal axes being mutually perpendicular to the axis of the detector element, the centerline of the aircraft being parallel with the outer ring of the gimbal mechanism and perpendicular to the outer gimbal axis, means carried by said outer ring for indicating when the plane of said outer ring is perpendicular to true vertical during initial adjustment of the gimbal mechanism to align said detector element axis with true vertical thereby to establish a rectangular coordinate system including true vertical and the centerline of the aircraft and from which to measure angular variations of the aircraft with respect to the axis of the detector element, a pair of potentiometers carried by said gimbal mechanism and respectively mounted for adjustment about said gimbal axes in response to movements of the inner and outer gimbal rings thereabout, and a pair of bridge circuits including said potentiometers and a pair of recording meters individual thereto for measuring and recording said angular variations.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,412,586 Welch Apr. 11, 1922 1,863,415 Rieber June 14, 1932 2,464,911 White Mar. 22, 1949 2,482,504 Pennington Sept. 20, 1949 2,488,389 Felch et a1 Nov. 15, 1949 2,519,718 Strobino Aug. 22, 1950 2,534,632 Smith Dec. 19, 1950 2,541,213 Davidson Feb. 13, 1951 2,555,209 Vacquier et al. May 29, 1951- 2,555,328 Esval et al. June 5, 1951 2,559,586 Bjarnason July 10, 1951 2,598,697 Jensen June 3, 1952 FOREIGN PATENTS Number Country Date 2,105 Great Britain Jan. 29, 1912 01 1911 

